ReTiF: A declarative real-time scheduling framework for POSIX systems

This paper proposes a novel framework providing a declarative interface to access real-time process scheduling services available in an operating system kernel. The main idea is to let applications declare their temporal requirements or characteristics without knowing exactly which underlying scheduling algorithms are offered by the system. The proposed framework can adequately handle such a set of heterogeneous requirements configuring the platform and partitioning the requests among the available multitude of cores, so to exploit the various scheduling disciplines that are available in the kernel, matching application requirements in the best possible way. The framework is realized with a modular architecture in which different plugins handle independently certain real-time scheduling features. The architecture is designed to make its behavior customization easier and enhance the support for other operating systems by introducing and configuring additional plugins

Flexible Scheduling in Middleware for Distributed rate-based real-time applications - Doctoral Dissertation, May 2002

Distributed rate-based real-time systems, such as process control and avionics mission computing systems, have traditionally been scheduled statically. Static scheduling provides assurance of schedulability prior to run-time overhead. However, static scheduling is brittle in the face of unanticipated overload, and treats invocation-to-invocation variations in resource requirements inflexibly. As a consequence, processing resources are often under-utilized in the average case, and the resulting systems are hard to adapt to meet new real-time processing requirements. Dynamic scheduling offers relief from the limitations of static scheduling. However, dynamic scheduling offers relief from the limitations of static scheduling. However, dynamic scheduling often has a high run-time cost because certain decisions are enforced on-line. Furthermore, under conditions of overload tasks can be scheduled dynamically that may never be dispatched, or that upon dispatch would miss their deadlines. We review the implications of these factors on rate-based distributed systems, and posits the necessity to combine static and dynamic approaches to exploit the strengths and compensate for the weakness of either approach in isolation. We present a general hybrid approach to real-time scheduling and dispatching in middleware, that can employ both static and dynamic components. This approach provides (1) feasibility assurance for the most critical tasks, (2) the ability to extend this assurance incrementally to operations in successively lower criticality equivalence classes, (3) the ability to trade off bounds on feasible utilization and dispatching over-head in cases where, for example, execution jitter is a factor or rates are not harmonically related, and (4) overall flexibility to make more optimal use of scarce computing resources and to enforce a wider range of application-specified execution requirements. This approach also meets additional constraints of an increasingly important class of rate-based systems, those with requirements for robust management of real-time performance in the face of rapidly and widely changing operating conditions. To support these requirements, we present a middleware framework that implements the hybrid scheduling and dispatching approach described above, and also provides support for (1) adaptive re-scheduling of operations at run-time and (2) reflective alternation among several scheduling strategies to improve real-time performance in the face of changing operating conditions. Adaptive re-scheduling must be performed whenever operating conditions exceed the ability of the scheduling and dispatching infrastructure to meet the critical real-time requirements of the system under the currently specified rates and execution times of operations. Adaptive re-scheduling relies on the ability to change the rates of execution of at least some operations, and may occur under the control of a higher-level middleware resource manager. Different rates of execution may be specified under different operating conditions, and the number of such possible combinations may be arbitrarily large. Furthermore, adaptive rescheduling may in turn require notification of rate-sensitive application components. It is therefore desirable to handle variations in operating conditions entirely within the scheduling and dispatching infrastructure when possible. A rate-based distributed real-time application, or a higher-level resource manager, could thus fall back on adaptive re-scheduling only when it cannot achieve acceptable real-time performance through self-adaptation. Reflective alternation among scheduling heuristics offers a way to tune real-time performance internally, and we offer foundational support for this approach. In particular, run-time observable information such as that provided by our metrics-feedback framework makes it possible to detect that a given current scheduling heuristic is underperforming the level of service another could provide. Furthermore we present empirical results for our framework in a realistic avionics mission computing environment. This forms the basis for guided adaption. This dissertation makes five contributions in support of flexible and adaptive scheduling and dispatching in middleware. First, we provide a middle scheduling framework that supports arbitrary and fine-grained composition of static/dynamic scheduling, to assure critical timeliness constraints while improving noncritical performance under a range of conditions. Second, we provide a flexible dispatching infrastructure framework composed of fine-grained primitives, and describe how appropriate configurations can be generated automatically based on the output of the scheduling framework. Third, we describe algorithms to reduce the overhead and duration of adaptive rescheduling, based on sorting for rate selection and priority assignment. Fourth, we provide timely and efficient performance information through an optimized metrics-feedback framework, to support higher-level reflection and adaptation decisions. Fifth, we present the results of empirical studies to quantify and evaluate the performance of alternative canonical scheduling heuristics, across a range of load and load jitter conditions. These studies were conducted within an avionics mission computing applications framework running on realistic middleware and embedded hardware. The results obtained from these studies (1) demonstrate the potential benefits of reflective alternation among distinct scheduling heuristics at run-time, and (2) suggest performance factors of interest for future work on adaptive control policies and mechanisms using this framework

Real-time scheduling algorithms, task visualization

Real-time systems are computer systems that require responses to events within specified time limits or constraints. Many real-time systems are digital control systems comprised entirely of binary logic or a microprocessor dedicated to one software application that is its own operating system. In recent years, the reliability of general-purpose real-time operating systems (RTOS) consisting of a scheduler and system resource management have improved. In this project, I write a real-time simulator, a workload generator, analysis tools, several test cases, and run and interpret results. My experiments focus on providing evidence to support the claim that for the Rate Monotonic scheduling algorithm (RM), workloads with harmonically non-similar, periodic tasks are more difficult to schedule. The analysis tool I have developed is a measurement system and real-time simulator that analyzes real-time scheduling strategies. I have also developed a visualization system to display the scheduling decisions of a real-time scheduler. Using the measurement and visualization systems, I investigate scheduling algorithms for real-time schedulers and compare their performance. I run different workloads to test the scheduling algorithms and analyze what types of workload characteristics are preferred for real-time benchmarks

Hard Real-Time Linux for Off-The-Shelf Multicore Architectures

This document describes the research results that were obtained from the development of a real-time extension for the Linux operating system. The paper describes a full extension of the kernel, which enables hard real-time performance on a 64-bit x86 architecture. In the first part of this study, real-time systems are categorized and concepts of real-time operating systems are introduced to the reader. In addition, numerous well-known real-time operating systems are considered. QNX Neutrino, RT_PREEMPT Linux Patch and HLRT Linux Patch are analyzed in detail. The core concepts of these systems are shown and discussed. Furthermore, a test suite is developed, which is used to obtain expressive benchmarks from the systems that were analyzed before. The systems are evaluated on the basis of these benchmarks and compared to the real-time extension which is developed in this work. A requirements catalogue is defined based on the analysis of the stated operating systems. The design of a real-time extension is developed based on the specification catalogue and the identified core concepts. Furthermore, the concrete implementation of the developed real-time extension is presented in detail. Finally, the benchmarks of all analyzed systems, including the developed real-time extension, are compared to each other and evaluated

Generalizing List Scheduling for Stochastic Soft Real-time Parallel Applications

Advanced architecture processors provide features such as caches and branch prediction that result in improved, but variable, execution time of software. Hard real-time systems require tasks to complete within timing constraints. Consequently, hard real-time systems are typically designed conservatively through the use of tasks? worst-case execution times (WCET) in order to compute deterministic schedules that guarantee task?s execution within giving time constraints. This use of pessimistic execution time assumptions provides real-time guarantees at the cost of decreased performance and resource utilization. In soft real-time systems, however, meeting deadlines is not an absolute requirement (i.e., missing a few deadlines does not severely degrade system performance or cause catastrophic failure). In such systems, a guaranteed minimum probability of completing by the deadline is sufficient. Therefore, there is considerable latitude in such systems for improving resource utilization and performance as compared with hard real-time systems, through the use of more realistic execution time assumptions. Given probability distribution functions (PDFs) representing tasks? execution time requirements, and tasks? communication and precedence requirements, represented as a directed acyclic graph (DAG), this dissertation proposes and investigates algorithms for constructing non-preemptive stochastic schedules. New PDF manipulation operators developed in this dissertation are used to compute tasks? start and completion time PDFs during schedule construction. PDFs of the schedules? completion times are also computed and used to systematically trade the probability of meeting end-to-end deadlines for schedule length and jitter in task completion times. Because of the NP-hard nature of the non-preemptive DAG scheduling problem, the new stochastic scheduling algorithms extend traditional heuristic list scheduling and genetic list scheduling algorithms for DAGs by using PDFs instead of fixed time values for task execution requirements. The stochastic scheduling algorithms also account for delays caused by communication contention, typically ignored in prior DAG scheduling research. Extensive experimental results are used to demonstrate the efficacy of the new algorithms in constructing stochastic schedules. Results also show that through the use of the techniques developed in this dissertation, the probability of meeting deadlines can be usefully traded for performance and jitter in soft real-time systems

Performance Analysis of Live-Virtual-Constructive and Distributed Virtual Simulations: Defining Requirements in Terms of Temporal Consistency

This research extends the knowledge of live-virtual-constructive (LVC) and distributed virtual simulations (DVS) through a detailed analysis and characterization of their underlying computing architecture. LVCs are characterized as a set of asynchronous simulation applications each serving as both producers and consumers of shared state data. In terms of data aging characteristics, LVCs are found to be first-order linear systems. System performance is quantified via two opposing factors; the consistency of the distributed state space, and the response time or interaction quality of the autonomous simulation applications. A framework is developed that defines temporal data consistency requirements such that the objectives of the simulation are satisfied. Additionally, to develop simulations that reliably execute in real-time and accurately model hierarchical systems, two real-time design patterns are developed: a tailored version of the model-view-controller architecture pattern along with a companion Component pattern. Together they provide a basis for hierarchical simulation models, graphical displays, and network I/O in a real-time environment. For both LVCs and DVSs the relationship between consistency and interactivity is established by mapping threads created by a simulation application to factors that control both interactivity and shared state consistency throughout a distributed environment

Towards a distributed real-time system for future satellite applications

Thesis (MScEng)--University of Stellenbosch, 2003.ENGLISH ABSTRACT: The Linux operating system and shared Ethernet are alternative technologies with the potential to reduce both the development time and costs of satellites as well as the supporting infrastructure. Modular satellites, ground stations and rapid proto typing testbeds also have a common requirement for distributed real-time computation. The identified technologies were investigated to determine whether this requirement could also be met. Various real-time extensions and modifications are currently available for the Linux operating system. A suitable open source real-time extension called Real-Time Application Interface (RTAI) was selected for the implementation of an experimental distributed real-time system. Experimental results showed that the RTAI operating system could deliver deterministic realtime performance, but only in the absence of non-real-time load. Shared Ethernet is currently the most popular and widely used commercial networking technology. However, Ethernet wasn't developed to provide real-time performance. Several methods have been proposed in literature to modify Ethernet for real-time communications. A token passing protocol was found to be an effective and least intrusive solution. The Real-Time Token (RTToken) protocol was designed to guarantee predictable network access to communicating real-time tasks. The protocol passes a token between nodes in a predetermined order and nodes are assigned fixed token holding times. Experimental results proved that the protocol offered predictable network access with bounded jitter. An experimental distributed real-time system was implemented, which included the extension of the RTAI operating system with the RTToken protocol, as a loadable kernel module. Real-time tasks communicated using connectionless Internet protocols. The Real-Time networking (RTnet) subsystem of RTAI supported these protocols. Under collision-free conditions consistent transmission delays with bounded jitter was measured. The integrated RTToken protocol provided guaranteed and bounded network access to communicating real-time tasks, with limit overheads. Tests exhibited errors in some of the RTAI functionality. Overall the investigated technologies showed promise in being able to meet the distributed real-time requirements of various applications, including those found in the satellite environment.AFRIKAANSE OPSOMMING: Die Linux bedryfstelsel en gedeelde Ethernet is geïdentifiseer as potensiële tegnologieë vir satelliet bedryf wat besparings in koste en vinniger ontwikkeling te weeg kan bring. Modulêr ontwerpte satelliete, grondstasies en ontwikkeling platforms het 'n gemeenskaplike behoefte vir verspreide intydse verwerking. Verskillende tegnologieë is ondersoek om te bepaal of aan die vereiste ook voldoen kan word. Verskeie intydse uitbreidings en modifikasies is huidiglik beskikbaar vir die Linux bedryfstelsel. Die "Real-Time Application Interface" (RTAI) bedryfstelsel is geïdentifiseer as 'n geskikte intydse uitbreiding vir die implementering van 'n eksperimentele verspreide intydse stelsel. Eksperimentele resultate het getoon dat die RTAI bedryfstelsel deterministies en intyds kan opereer, maar dan moet dit geskied in die afwesigheid van 'n nie-intydse verwerkingslas. Gedeelde Ethernet is 'n kommersiële network tegnologie wat tans algemeen beskikbaar is. Die tegnologie is egter nie ontwerp vir intydse uitvoering nie. Verskeie metodes is in die literatuur voorgestelom Ethernet te modifiseer vir intydse kommunikasie. Hierdie ondersoek het getoon dat 'n teken-aangee protokol die mees effektiewe oplossing is en waarvan die implementering min inbreuk maak. Die "Real-Time Token" (RTToken) protokol is ontwerp om voorspelbare netwerk toegang tot kommunikerende intydse take te verseker. Die protokol stuur 'n teken tussen nodusse in 'n voorafbepaalde volgorde. Nodusse word ook vaste teken hou-tye geallokeer. Eksperimentele resultate het aangedui dat die protokol deterministiese netwerk toegang kan verseker met begrensde variasies. 'n Eksperimentele verspreide intydse stelsel is geïmplementeer. Dit het ingesluit die uitbreiding van die RTAI bedryfstelsel met die RTToken protokol; verpak as 'n laaibare bedryfstelsel module. Intydse take kan kommunikeer met verbindinglose protokolle wat deur die "Real-Time networking" (RTnet) substelsel van RTAI ondersteun word. Onder ideale toestande is konstante transmissie vertragings met begrensde variasies gemeet. Die integrasie van die RTToken protokol het botsinglose netwerk toegang aan kommunikerende take verseker, met beperkte oorhoofse koste as teenprestasie. Eksperimente het enkele foute in die funksionaliteit van RTAI uitgewys. In die algemeen het die voorgestelde tegnologieë getoon dat dit potensiaal het vir verskeie verspreide intydse toepassings in toekomstige satelliet en ook ander omgewings

Real-time operating system support for multicore applications

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia de Automação e Sistemas, Florianópolis, 2014Plataformas multiprocessadas atuais possuem diversos níveis da memória cache entre o processador e a memória principal para esconder a latência da hierarquia de memória. O principal objetivo da hierarquia de memória é melhorar o tempo médio de execução, ao custo da previsibilidade. O uso não controlado da hierarquia da cache pelas tarefas de tempo real impacta a estimativa dos seus piores tempos de execução, especialmente quando as tarefas de tempo real acessam os níveis da cache compartilhados. Tal acesso causa uma disputa pelas linhas da cache compartilhadas e aumenta o tempo de execução das aplicações. Além disso, essa disputa na cache compartilhada pode causar a perda de prazos, o que é intolerável em sistemas de tempo real críticos. O particionamento da memória cache compartilhada é uma técnica bastante utilizada em sistemas de tempo real multiprocessados para isolar as tarefas e melhorar a previsibilidade do sistema. Atualmente, os estudos que avaliam o particionamento da memória cache em multiprocessadores carecem de dois pontos fundamentais. Primeiro, o mecanismo de particionamento da cache é tipicamente implementado em um ambiente simulado ou em um sistema operacional de propósito geral. Consequentemente, o impacto das atividades realizados pelo núcleo do sistema operacional, tais como o tratamento de interrupções e troca de contexto, no particionamento das tarefas tende a ser negligenciado. Segundo, a avaliação é restrita a um escalonador global ou particionado, e assim não comparando o desempenho do particionamento da cache em diferentes estratégias de escalonamento. Ademais, trabalhos recentes confirmaram que aspectos da implementação do SO, tal como a estrutura de dados usada no escalonamento e os mecanismos de tratamento de interrupções, impactam a escalonabilidade das tarefas de tempo real tanto quanto os aspectos teóricos. Entretanto, tais estudos também usaram sistemas operacionais de propósito geral com extensões de tempo real, que afetamos sobre custos de tempo de execução observados e a escalonabilidade das tarefas de tempo real. Adicionalmente, os algoritmos de escalonamento tempo real para multiprocessadores atuais não consideram cenários onde tarefas de tempo real acessam as mesmas linhas da cache, o que dificulta a estimativa do pior tempo de execução. Esta pesquisa aborda os problemas supracitados com as estratégias de particionamento da cache e com os algoritmos de escalonamento tempo real multiprocessados da seguinte forma. Primeiro, uma infraestrutura de tempo real para multiprocessadores é projetada e implementada em um sistema operacional embarcado. A infraestrutura consiste em diversos algoritmos de escalonamento tempo real, tais como o EDF global e particionado, e um mecanismo de particionamento da cache usando a técnica de coloração de páginas. Segundo, é apresentada uma comparação em termos da taxa de escalonabilidade considerando o sobre custo de tempo de execução da infraestrutura criada e de um sistema operacional de propósito geral com extensões de tempo real. Em alguns casos, o EDF global considerando o sobre custo do sistema operacional embarcado possui uma melhor taxa de escalonabilidade do que o EDF particionado com o sobre custo do sistema operacional de propósito geral, mostrando claramente como diferentes sistemas operacionais influenciam os escalonadores de tempo real críticos em multiprocessadores. Terceiro, é realizada uma avaliação do impacto do particionamento da memória cache em diversos escalonadores de tempo real multiprocessados. Os resultados desta avaliação indicam que um sistema operacional "leve" não compromete as garantias de tempo real e que o particionamento da cache tem diferentes comportamentos dependendo do escalonador e do tamanho do conjunto de trabalho das tarefas. Quarto, é proposto um algoritmo de particionamento de tarefas que atribui as tarefas que compartilham partições ao mesmo processador. Os resultados mostram que essa técnica de particionamento de tarefas reduz a disputa pelas linhas da cache compartilhadas e provê garantias de tempo real para sistemas críticos. Finalmente, é proposto um escalonador de tempo real de duas fases para multiprocessadores. O escalonador usa informações coletadas durante o tempo de execução das tarefas através dos contadores de desempenho em hardware. Com base nos valores dos contadores, o escalonador detecta quando tarefas de melhor esforço o interferem com tarefas de tempo real na cache. Assim é possível impedir que tarefas de melhor esforço acessem as mesmas linhas da cache que tarefas de tempo real. O resultado desta estratégia de escalonamento é o atendimento dos prazos críticos e não críticos das tarefas de tempo real.Abstracts: Modern multicore platforms feature multiple levels of cache memory placed between the processor and main memory to hide the latency of ordinary memory systems. The primary goal of this cache hierarchy is to improve average execution time (at the cost of predictability). The uncontrolled use of the cache hierarchy by realtime tasks may impact the estimation of their worst-case execution times (WCET), specially when real-time tasks access a shared cache level, causing a contention for shared cache lines and increasing the application execution time. This contention in the shared cache may leadto deadline losses, which is intolerable particularly for hard real-time (HRT) systems. Shared cache partitioning is a well-known technique used in multicore real-time systems to isolate task workloads and to improve system predictability. Presently, the state-of-the-art studies that evaluate shared cache partitioning on multicore processors lack two key issues. First, the cache partitioning mechanism is typically implemented either in a simulated environment or in a general-purpose OS (GPOS), and so the impact of kernel activities, such as interrupt handlers and context switching, on the task partitions tend to be overlooked. Second, the evaluation is typically restricted to either a global or partitioned scheduler, thereby by falling to compare the performance of cache partitioning when tasks are scheduled by different schedulers. Furthermore, recent works have confirmed that OS implementation aspects, such as the choice of scheduling data structures and interrupt handling mechanisms, impact real-time schedulability as much as scheduling theoretic aspects. However, these studies also used real-time patches applied into GPOSes, which affects the run-time overhead observed in these works and consequently the schedulability of real-time tasks. Additionally, current multicore scheduling algorithms do not consider scenarios where real-time tasks access the same cache lines due to true or false sharing, which also impacts the WCET. This thesis addresses these aforementioned problems with cache partitioning techniques and multicore real-time scheduling algorithms as following. First, a real-time multicore support is designed and implemented on top of an embedded operating system designed from scratch. This support consists of several multicore real-time scheduling algorithms, such as global and partitioned EDF, and a cache partitioning mechanism based on page coloring. Second, it is presented a comparison in terms of schedulability ratio considering the run-time overhead of the implemented RTOS and a GPOS patched with real-time extensions. In some cases, Global-EDF considering the overhead of the RTOS is superior to Partitioned-EDF considering the overhead of the patched GPOS, which clearly shows how different OSs impact hard realtime schedulers. Third, an evaluation of the cache partitioning impacton partitioned, clustered, and global real-time schedulers is performed.The results indicate that a lightweight RTOS does not impact real-time tasks, and shared cache partitioning has different behavior depending on the scheduler and the task's working set size. Fourth, a task partitioning algorithm that assigns tasks to cores respecting their usage of cache partitions is proposed. The results show that by simply assigning tasks that shared cache partitions to the same processor, it is possible to reduce the contention for shared cache lines and to provideHRT guarantees. Finally, a two-phase multicore scheduler that provides HRT and soft real-time (SRT) guarantees is proposed. It is shown that by using information from hardware performance counters at run-time, the RTOS can detect when best-effort tasks interfere with real-time tasks in the shared cache. Then, the RTOS can prevent best effort tasks from interfering with real-time tasks. The results also show that the assignment of exclusive partitions to HRT tasks together with the two-phase multicore scheduler provides HRT and SRT guarantees, even when best-effort tasks share partitions with real-time tasks